Currently available shRNA vectors and libraries have limited utility for in vivo studies of antiviral T cell responses. This hampers rapid identification of biological targets in vivo that could be manipulated therapeutically in the context of vaccines and adoptive immunotherapy to treat viral infections successfully. The overarching goals of Core B are to clone novel and superior shRNAmirs that cover the mouse genome more comprehensively than any other currently available tools, to do so in a vector that is useful for extended in vivo studies of adoptively transferred lymphocytes in the context of viral infections, and to make them available immediately to laboratories conducting Projects 1-3 of this U19 proposal, and ultimately to the research community at large. Our specific objectives are to clone two novel comprehensive gene-family shRNA libraries in a retroviral vector suitable for in vivo analysis of the murine immune system (Aim 1); to produce ready-to-transfect retroviral-shRNAmir DNA from cloned libraries for virus production (Aim 2); and, to clone smaller focused retroviral-shRNAmir sets and build custom delivery constructs (Aim 3), to facilitate specific follow-up studies outlined in Research Projects 1-3.